1. Field of the Invention
The present invention relates to an embedded touch screen.
2. Description of the Related Art
A tablet-type information terminal and a multifunctional mobile phone (so-called smartphone) including a touch screen have become popular. The “touch screen” as used herein refers to a device obtained by combining a touch panel to an image display device such as a liquid crystal display device. Further, the “touch panel” as used herein refers to a planar contact-detection type pointing device. A user directly touches the touch screen with a finger, a stylus, or the like while looking at an image displayed on the screen, thereby performing input.
The generally popularized touch screen is obtained by integrally bonding the image display device and the contact-detection type input device, which are separately formed. Such a touch panel has disadvantages in panel thickness and weight, the number of components, and its cost. Further, the number of manufacturing steps increases, and hence there arises a problem of decreasing yields.
In view of this, there has been proposed a touch screen in which the image display device and the contact-detection type input device are not separately formed but integrally formed. Such a touch screen is referred to as, for example, “embedded type” or “in-cell type”. In this specification, such a touch screen is hereinafter called an “embedded touch screen”.
JP 2008-027292 A describes an embedded touch screen in which an optical sensor and a thin film transistor (TFT) for taking out an output of the sensor as a charge voltage are arranged in a pixel of the liquid crystal display device.
JP 2011-527787 A describes an embedded touch screen in which a common electrode of the liquid crystal display device is divided into blocks for mutual capacitive coupling, and contact detection is performed based on the capacitive change during touching.
Eiji Kanda, Tsukasa Eguchi, Yasunori Hiyoshi, Taketo Chino, Yasushi Tsuchiya, Takahiro Iwashita, Tokuro Ozawa, Takao Miyazawa, Tomotaka Matsumoto (2008), “Integrated Active Matrix Capacitive Sensors for Touch Panel LIPS-TFTLCDs,” SID symposium digest of technical papers, Vol. 39, Issue 1, 834-837 describes an embedded touch screen in which a capacitive element and a signal reading TFT are provided in a pixel of the liquid crystal display device, and the capacitive fluctuation caused by the fluctuation of a liquid crystal cell gap during touching is read outside.
In the embedded touch screens described in JP 2008-027292 A and Kanda et al., a TFT different from a TFT for driving liquid crystal needs to be provided in the pixel, but in this case, the pixel aperture ratio reduces. Particularly in a small-sized or high resolution image display device, the TFT originally occupies a large area in proportion to the entire pixel, and hence it is not very realistic to further increase the number of TFTs in the pixel.
In the embedded touch screen described in JP 2011-527787 A, the common electrode needs to be divided into complicated shapes, and the contact detection is performed for each block, which degrades the accuracy of coordinate detection. In order to increase the accuracy of coordinate detection, it is necessary to divide the common electrode into a larger number of small blocks, but on the other hand, each block requires a detection circuit including an amplifier. Therefore, when the accuracy of coordinate detection is increased, the circuit scale of the detection circuit is increased.